A number of prior references which are of background interest relative to the present invention appear in a numbered list toward the end of the present specification and certain of these articles will be referenced, using the numbers of the listed references, in brackets, in this specification.
Radar systems have been employed in both non-military and military applications for a number of years. In recent years, the technical advances required in radar systems for the military have been considerable. When first employed in World War II, the accuracy and speed of radar were not overly critical. The radar was used to detect slow-moving ships and planes which could then be engaged on a not-too-critical time basis. More recently, speed and accuracy have become critical in order to allow the timely detection and destruction of smaller, fast-moving missiles having high destructive power. In addition, these contemporary radar systems are required to perform in an environment filled with highly efficient countermeasure devices.
Because of its resistance to jamming and interference from outside sources, and its superior range and angular accuracy, light has replaced electro-magnetic energy in many applications including communications and measuring systems. Laser rangefinder techniques have been successfully shown to determine the ranges of targets at distances up to five miles with accuracy of 2 to 10 meters, see reference [1], listed at the end of this specification. As with conventional radar systems, laser systems may be classified into two basic categories: (1) direct, or incoherent detection, and (2) heterodyne or coherent detection.
The theory of the former dictates that the best signal-to-noise ratio is provided when the transmitted energy is concentrated into the shortest possible pulse [2]. This yields a good range measuring and resolution capability. On the other hand, coherent detection requires highest possible average transmitted power for the best signal-to-noise ratio, irrespective of pulsewidth. In addition, accurate target radial velocity measurements can be obtained in the latter case. Woodward [3] pointed out, during the early stages of modern radar technology, that radar resolution and accuracy were functions of the signal bandwidth, being AM or FM in nature, regardless of the transmitter waveform. Thus a continuous power, or long pulse mode of operation heterodyne system, may also yield good range measuring and resolution capability when using a wideband signal. One technique for obtaining a wideband signal is to frequency modulate the long pulse, and this has been termed a "chirped" signal, see reference [9], and the associated receiver includes matched delay vs. frequency components to compress the return pulses. A more complex receiver is required to extract the wide band information available from this type of reflected signal, as opposed to the direct detection system. These receivers, as used in conventional radar systems, are designated as matched-filter signal processing receivers. The advantages of such signal processing techniques in radars, as pointed out by Cook [4], are:
1. More efficient use of the average power available at the transmitter. PA1 2. Increased system accuracy, both in ranging and velocity measurements. PA1 3. Reduction of jamming vulnerability.
In a recent publication [5], a laser range finder using heterodyne detection and chirp pulse compression is described. The wideband signal consists of a linear FM chirp pulse of relative long duration. The matched-filter at the receiver end is a Surface Acoustic Wave (SAW) device, which compresses the relative long FM chirp pulse into a narrow one (of the same bandwidth), from which the range and velocity information may be extracted. The duration of the compressed pulse is approximately the inverse of the bandwidth of the original signal. Thus, as the amount of frequency that is chirped increases, so does the resolution of the range and velocity measurements. In the above work, as described in greater detail hereinafter, an acoustooptic modulator, placed at the output of the laser, was used to obtain a chirp width of 14 MHz. This would yield a compressed pulse of the order of 100 ns. Further increases in frequency deviation, with attendant reduction in pulse width, are limited by transit time of the acoustic wave across the optical beam.
Hulme et al (Optical and Quantum Electronics 13, 1981, 35-45), reference [5], for example, have demonstrated a CO.sub.2 laser range finder using FM chirp modulation and pulse compression. They modulate the laser outside the laser cavity using acoustic optic (AO) modulation. It is an object of the present invention to provide an order of magnitude increase in range resolution because of wider frequency deviation employing an electro-optic (EO) modulator and a high-pressure CO.sub.2 laser.
Stein (IEEE J. of Quantum Electronics, August 1975, 630-31) previously used intracavity EO modulation to chirp a high-pressure laser in a radar system that does not use pulse compression. However, he only achieved a 20 MHz chirp with a pulse having a linear ramp during a portion of the pulse. He makes no mention of the specific percent linearity achieved. From the graphical data he provides, however, it appears that the linearity was poor, and it is apparent that an attempt at pulse compression of the type described herein would not have been successful. This difficulty with precise linearity was also encountered by Hulme and Collins [Society of Photo-Optical Instrumentation Engineers' Proceedings; Vol. 236, pp. 135-138]. It is, in fact, this finding that compelled them to use the acousto-optic modulation as described above. It is another object of the present invention to provide high linearity frequency modulation of about 100 MHz thus making usable, broad-band, pulse compression possible.
More generally, it is an object of the present invention to provide a laser radar which overcomes the limitations of the prior art described above.